A passageway system or pipe system which is connected to a noise source, for example in exhaust system mounted to an internal combustion engine, is generally made up of a plurality of passageways or pipes together with one or more mufflers. From an acoustic point of view these parts make up one or more so called mass-spring systems, where each muffler acts as a spring and in which the air or the gas in the pipe acts as the mass. The spring stiffness is thus directly proportional to the square of the sound velocity for the gas medium which is present and inversely proportional to the volume of the muffler. The mass for respective passageway sections is directly proportional to the length of the passageway divided by its area.
For an exhaust system made up of a muffler and an inlet and outlet passageway a so-called system resonant frequency will arise. At this resonant frequency the so-called input damping is negative, i.e. sound pulses from, for example, exhaust valve openings exit the system as amplified sound.
For internal combustion engines in motor vehicles it is desired that this resonant frequency lies below the ignition frequency of the engine when idling. For a 4-cylinder engine with an idling speed of 750 rpm this means that a first resonant frequency of the system considerably below 25 Hz is sought. This low resonant frequency is possible to achieve with the help of a very large muffler and a moderate passageway length downstream of the muffler. Due to space considerations, vehicle design restricts the possibility of using a large muffler and so in practice it is advantageous to obtain low resonant frequency by placing the muffler well upstream in the system so that the length of the passageway downstream of the muffler can be increased by a sufficient amount.
With increased passageway length downstream of the muffler the risk increases that so-called standing sound waves of half the wave length (or multiples thereof) in frequency coincide with the ignition frequency of the engine or multiples thereof at various engine speeds.
With a pipe length of, for example, 1.4 m downstream of the muffler and normal exhaust temperatures of, with normal driving, around 200.degree. C., strong resonances arise due to the so-called first standing wave (so-called "half lambda") at 160 Hz.
For a 5-cylinder engine the first standing wave coincides with the firing frequency at approximately 4000 rpm, with double ignition frequency at approximately 2000 rpm and trip ignition frequency at around 1300 rpm. This results in greatly increased noise amplification.
With higher exhaust temperatures, i.e. when the engine is under greater load, corresponding resonance amplification occurs at higher engine speeds.
At full load when the exhaust gas temperature in the exhaust system's latter section often exceeds 700.degree. C., the resonance amplification is delayed until almost double engine speed. The second so-called standing wave, i.e. when the full sound wave length coincides with 1.4 m pipe length, gives double resonant frequency 32 Hz. In other words resonance amplification during normal driving arises even at 8000 rpm with the ignition frequency, at 4000 rpm with the double ignition frequency and at 2600 rpm with the triple ignition frequency. These latter resonance amplifications of the ignition frequency and their multiples are generally somewhat milder.
An alternative way of achieving a low first resonant frequency of the system is to add a further muffler. Such a system is shown in, for example, U.S. Pat. No. 4,537,278, FIG. 1. A double mass-spring system does however give rise to a second system resonant frequency. This normally arises at around 70 to 120 Hz. Further disadvantages with this solution are that it is relatively expensive to produce and its weight is increased.
In U.S. Pat. No. 3,316,812 and U.S. Pat. No. 3,415,338 so-called quarter-wave tubes are used to reduce the disadvantages with the one muffler system's standing wave phenomena of the half wave length or its multiples (.lambda., 1.5.lambda., 2.lambda., etc.) arising in pipe sections located between the free chambers as well as between mufflers or between the muffler and the exhaust outlet.
These alternative solutions have the common disadvantage that the temperature in the quarter-wave tubes normally differs greatly from the temperature in the exhaust system passageway.
This means that the quarter-wave tube's constant length corresponds to one of .lambda.2, 1.5.lambda. etc., in the pipe 16 only within a very restricted exhaust gas temperature range.
Since a so-called quarter-wave tube of traditional form has very narrowband damping characteristics, these alternative solutions present large restrictions in the muffler properties for the ignition pulse frequency and its multiples.
These disadvantages will be apparent from the following examples:
If it is an object to dampen the resonance amplification of any of the engine's pulse frequencies (the basic ignition frequency and its multiples) which are associated with the first standing wave, according to U.S. Pat. No. 3,396,812 the quarter-wave tube should be employed at the maximum pressure region for the standing wave. This means that the connection of the tube should be placed in the central region of the pipe.
If the quarter-wave tube's transverse dimension corresponds to the transverse dimension of the pipe, a very effective sound dampening of the half-wave resonant frequency is obtained in a known manner. This however applies only within a very restricted frequency range. It is furthermore known that amplifications of other frequencies arise due to the so-called sideband effects. With exemplified length and transverse dimensions these amplifications are achieved in a known manner at about 0.7 times the original resonant frequency and at about 1.4 times the original resonant frequency.
If, with 5-cylinder engines, sound damping of second order firing pulse frequency is desired, at the same engine speed a strong amplification of third order ignition pulse frequency is obtained.
Since the quarter-wave tube according to the example is externally located, temperature differences of up to 600.degree. to 700.degree. C. between the tube and the exhaust pipe arise.
If for example an external quarter-wave pipe's length is adapted so as to be effective at an exhaust pipe temperature of 200.degree. C., under full load an incorrect frequency correlation of up to a factor of 1.8 is caused. When idling and under part-load the error in the correlation can be up to a factor of 0.6.
Incorrect correlation can result in both that the necessary damping is not achieved and that sound which lies near to the ignition frequency or its multiples is amplified. This can arise because of the sideband amplifications at frequencies coinciding with the ignition frequency or its multiples.